There is evidence that the microcirculation undergoes structural vascular adaptations and changes in responsiveness in hypertension. It is the goal of the proposed research to evaluate the effects of hypertension on the microcirculation, and to delineate the participation of the precapillary resistance vessels in the development of hypertension. These problems will be studied first in the cremaster muscle of the spontaneously hypertensive rat (SHR) and then expanded to other hypertensive models and mesentery preparations for comparison. The research project will determine if there are reduced numbers of small arterioles in the SHR, if there is an increased wall/lumen ratio, and if the changes found are the causes or effect of hypertension. Mechanisms of increased peripheral resistance will be studied in other hypertensive rat models to separate hereditary changes from more universal alterations. Measurements of red cell velocity (V) will be made in arterioles, capillaries, and venules by an on-line and video, off-line dual-slit photometric method. Arteriolar blood flow (Q) will be calculated from the Q equals V pi D-squared/6.4, where D is the lumen diameter measured from a video monitor. Capillary density and wall thickness will also be determined from the video monitor. Microvascular blood pressure measurements will be made with a servo-null pressure system. The data will be analyzed by grouping the vessels according to branching order and comparing the measurements in SHR to those found in Wistar-Kyoto rats.